1. Field of the Invention
The present invention relates to a thermoplastic molding material comprising polystyrene containing a halogen-containing flame retardant, optionally a blowing agent, and 2,2-bis(4-allyloxy-3,5-dibromophenyl)-propane, which is a synergist, present in an amount of from 0.01 weight percent to 1.0 weight percent based on the weight of the polystyrene.
2. Description of the Prior Art
Halogen-containing organic compounds are effective flame retardants for polystyrene. However, the relatively large quantities of halogen compounds which must be added to the polymers in order to achieve sufficient flame retardation have a negative influence on many physical properties of the polymers.
The flame retarding effect of halogen compounds can be increased by additives having synergistic effects, which considerably reduce the amount needed for the flame retarding effect. Known and commonly used synergists include organic peroxides such as dicumyl peroxide. These synergists are toxic and at times decompose in an explosive manner. The handling and storage of these substances therefore requires expensive safety measures. Furthermore, even at room temperature peroxides slowly decompose so that the effectiveness of the flame retardation of peroxide containing polystyrene can decrease when they are stored for a long period of time. The decomposition of the peroxides increases with rising temperature. Peroxide synergists, therefore, cannot be used for the flame retardation of polystyrene which is processed at relatively high temperatures. For the foam extrusion of polystyrene, which is carried out at temperatures around 200.degree. C., peroxide synergists cannot be used.
Other familiar synergists such as organic compounds having instable C--C-- or N--N-bonds decompose during prolonged storage or when exposed to increased temperatures.
The synergist of this invention is a known compound. In the literature, it is referred to as "tetrabromodiandiallylether" or as "diallylether of tetrabromobisphenol A". British Pat. No. 1,206,171 and German Published application No. 2,226,694 describe the use of this bromine compound as flame retardant for plastics, for instance, also for polystyrene. However, these patents do not recommend the use of 2,2-bis-(4-allyloxy-3,5-dibromo- phenyl)-propane together with other halogen compounds and a possible synergistic effect is not mentioned at all.
An object of this invention was to develop a flame-retardant synergist for polystyrene containing a halogen-containing flame retardant and optionally a blowing agent which does not have the drawbacks of known peroxide synergists, and which, in particular, is stable when exposed to elevated temperatures.